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Topic: Hot tube - Good to have? (Read 646 times)

Well this thread is the result of one of my long term experiments on the exhaust with and without the hot tube aka cat con.While I had it removed around a year ago, upjetted the carb(BS 32) with a 17.5 pilot, main was already at 120.Immediate observation was the increased roar of the exhaust but I could see that the motorcycle response at speeds less than 30 was much more strangled.The bike would shudder on speeds around 15-30kmph, 1st through 3rd gears.During last October's ride in the Himalayas, again felt the lack of low speed torque which otherwise is in plenty with the Electra.

So, after about an year, today, got the hot tube welded back. Voila, the shuddering has almost disappeared and the throttle response is much more linear/smooth.

Throughout, the exhaust has been a glass wool filled goldstar mimic.I would want to know if all of you guys who removed this tube had pleasant experiences and how exactly this tube effects the ride quality(most comments I read in the forum here say that removing it is better but i feel otherwise).

From what I've read here, the hot tube is the most restrictive thing in the exhaust system... silencers don't add or subtract much flowability, but removing the hot tube is one of the only ways to give more breathability. I removed my hot tube, installed a K&N filter (open to the air) and experimented with jets and needle settings and the mixture screw.

I have the BS29, so my settings won't be relevant to yours, and I also think I'm running a little rich. From what I understand, the low end torque is based on a combination of the air/fuel mixture screw and the needle height- main jet doesn't come into play until higher throttle ranges. When I removed the hot pipe and went to a glasspack muffler like yours I also had some similar symptoms of starved throttle ranges, and it took some experimentation to get it to a workable setting.

I have the BS29 and a 17.5 pilot, 122.5 main (probably too rich), and my needle is at the richest setting then raised even more by a tiny washer (shimmed it up). Idle mixture screw is 1/2 turn out. I think the change that yielded me the most power was raising the needle height. When I removed the hot pipe and added the free flowing intake I needed to add more fuel into the mixture, and now that I've done that it has a lot more power than it ever had before.

Sorry for the rambling but I was bored. Hope this offers some help. It's a great bike without the free-flow mods, and it's a great bike after doing the mods. Takes some time to get it set up right, and it might not pull as smoothly as it did stock, but it'll have much more growl and power once it's done successfully.Best of luck!

Well this thread is the result of one of my long term experiments on the exhaust with and without the hot tube aka cat con.While I had it removed around a year ago, upjetted the carb(BS 32) with a 17.5 pilot, main was already at 120.Immediate observation was the increased roar of the exhaust but I could see that the motorcycle response at speeds less than 30 was much more strangled.The bike would shudder on speeds around 15-30kmph, 1st through 3rd gears.During last October's ride in the Himalayas, again felt the lack of low speed torque which otherwise is in plenty with the Electra.

So, after about an year, today, got the hot tube welded back. Voila, the shuddering has almost disappeared and the throttle response is much more linear/smooth.

Throughout, the exhaust has been a glass wool filled goldstar mimic.I would want to know if all of you guys who removed this tube had pleasant experiences and how exactly this tube effects the ride quality(most comments I read in the forum here say that removing it is better but i feel otherwise).

My only comment is you gave up too soon. When you removed the hot tube you didn't continue to follow through on getting your carb well tuned in. Removing the hot tube is a drastic change (+ you totally changed the carburetor) & it's not a simple matter getting the carb straightened out to play nicely with the engine & your exhaust changes. It takes patience, perseverance & a lot of tearing your hair out moments & frustration. You'll never hit the right combination of pilot, main, needle height & fuel/air mixture screw the first time. It takes many times of carb off, carb on, plug out, plug in & experimenting. If a person's not ready to commit to that, their better choice might be to leave everything alone.

Here's some information.The hot tube is at the very end of the header pipe. This means that it cannot help speed up the exhaust speeds which would help cylinder scavenging because it is too far from the exhaust port. Therefore it can only be a restriction which hampers the scavenging of the cylinder, and that can never improve performance. If the restriction was at the joint of exhaust port and header pipe, then I might see that there is some potential for "improvement" at low rpms because of improved extraction effect at low rpms. However, with the location of the hot tube, that is not possible, so we have a tuning problem that seems to be running better with increased "back pressure", and that is almost always related to incorrect carburetor tuning. Specifically, a too-lean condition at the rpms showing the improvement with the hot tube in place.

It is okay and fine if you like this combination better, and essentially you have compensated for the way you tuned the carburetor by increasing the exhaust restriction again, which balanced out the mixture errors.

I don't really endorse this type of "tuning" but it can be made to work okay like that and not hurt anything except performance, so I am not going to imply that you shouldn't do it. I will just say that further improvement is possible with the more typical tuning methods that most others employ.

Perhaps you are right. But this bike is my only means of commute. That's is why I cannot have it in the garage for long. So when I get an alternate ride, I intend to spend much more time and energy on making this bike a better tourer.

Here's some information.The hot tube is at the very end of the header pipe. This means that it cannot help speed up the exhaust speeds which would help cylinder scavenging because it is too far from the exhaust port. Therefore it can only be a restriction which hampers the scavenging of the cylinder, and that can never improve performance. If the restriction was at the joint of exhaust port and header pipe, then I might see that there is some potential for "improvement" at low rpms because of improved extraction effect at low rpms. However, with the location of the hot tube, that is not possible, so we have a tuning problem that seems to be running better with increased "back pressure", and that is almost always related to incorrect carburetor tuning. Specifically, a too-lean condition at the rpms showing the improvement with the hot tube in place.

I don't really endorse this type of "tuning" but it can be made to work okay like that and not hurt anything except performance, so I am not going to imply that you shouldn't do it. I will just say that further improvement is possible with the more typical tuning methods that most others employ.

Makes sense Tom and I too dint like this reversal. But as I said, its the only means of commute and I could not tolerate the s%$%y ride quality anymore. So will wait until I get a long vacation or an alternate ride. Your explanation helps a lot!

From what I've read here, the hot tube is the most restrictive thing in the exhaust system... silencers don't add or subtract much flowability, but removing the hot tube is one of the only ways to give more breathability. I removed my hot tube, installed a K&N filter (open to the air) and experimented with jets and needle settings and the mixture screw.

I have the BS29, so my settings won't be relevant to yours, and I also think I'm running a little rich. From what I understand, the low end torque is based on a combination of the air/fuel mixture screw and the needle height- main jet doesn't come into play until higher throttle ranges. When I removed the hot pipe and went to a glasspack muffler like yours I also had some similar symptoms of starved throttle ranges, and it took some experimentation to get it to a workable setting.

I have the BS29 and a 17.5 pilot, 122.5 main (probably too rich), and my needle is at the richest setting then raised even more by a tiny washer (shimmed it up). Idle mixture screw is 1/2 turn out. I think the change that yielded me the most power was raising the needle height. When I removed the hot pipe and added the free flowing intake I needed to add more fuel into the mixture, and now that I've done that it has a lot more power than it ever had before.

Sorry for the rambling but I was bored. Hope this offers some help. It's a great bike without the free-flow mods, and it's a great bike after doing the mods. Takes some time to get it set up right, and it might not pull as smoothly as it did stock, but it'll have much more growl and power once it's done successfully.Best of luck!

Please dont be sorry! I love hearing people talk about the enfields, mods or no mods I do have some plans of shedding weight, lighter fuel tank, better air filter - a bike that is just meant to tour. Then, I shall work on the exhaust aspect as well.Am all ears for the arrival of the KTM 390 tourer, after which the AVL will undergo the surgery.

Good to hear- and I think you have the right approach. Just 'undo' the modification that caused the problem, and when you have time/patience/money/all-of-the-above get at it with some performance upgrades!

Good to hear- and I think you have the right approach. Just 'undo' the modification that caused the problem, and when you have time/patience/money/all-of-the-above get at it with some performance upgrades!

Thanks Superchuck.The combination of all three would have to wait for some time

I am curious - how about the other bikes, be it the Nortons, Triumphs or the Honda, Kawasaki, how radically different is their exhaust design?Do they have any hot tube as such? Are the owners worried about the 'back-pressure'? I know its a bit of an OT but some knowledge would help us understand our bikes better.

I am curious - how about the other bikes, be it the Nortons, Triumphs or the Honda, Kawasaki, how radically different is their exhaust design?Do they have any hot tube as such? Are the owners worried about the 'back-pressure'? I know its a bit of an OT but some knowledge would help us understand our bikes better.

It all depends on what year the bikes are, what country they're for, etc., etc.

All these things that are added to or stuck onto or into or hung off of an otherwise bare naked, decent-running motorcycle engine are government, and in some cases State, mandated.

ALL motorcycles sold here in the U.S. are buried up to their eyeballs (headlight rims) in Federal & State regulations. This has been true for what? 40 years?